1. Field of the Invention
The present invention relates to a device for assisting the piloting or the simulation of the piloting of a vehicle. It applies mainly to aircraft, but it can be applied also to all sorts of other air, land or sea vehicles, especially when they are manoeuvred in a three-dimensional space. The invention will be described with regard to the piloting of an aircraft, and in particular the piloting in the landing phases.
2. Discussion of the Background
Under manual piloting, the pilot acts by sight to modify the direction of movement of his vehicle for the aid of the trim and engine controls.
Assistance with the manual piloting of an aircraft can be carried out by displaying, in front of the pilot's eyes, symbols representing the terrestrial environment and the movement of this aircraft, these symbols being superimposed, when the visibility is sufficient, with the real horizon and real environment seen by the pilot through the windscreen of his vehicle. The position and shape of the symbols are computed and displayed for example by the computer controlling a head-up collimator from data supplied by sensors carried on board the aircraft.
On the display screen of a known device for assisting aircraft piloting, the artificial horizon computed is symbolized by a line which tilts as a function of the lateral tilt (angle of roll) of the aircraft. It is superimposed on the visible real horizon if visibility is sufficient. It replaces it when visibility is insufficient. The projection at infinity of the longitudinal axis of the aircraft is portrayed by a symbol termed the "aircraft symbol". This symbol is above the horizon line to a greater or lesser extent depending on whether the aircraft is nose-up to a greater or lesser extent (greater or lesser longitudinal attitude of the aircraft); the attitude of the aircraft can be referenced by the position of the aircraft symbol in front of an attitude scale perpendicular to the horizon line. The lateral position of the aircraft symbol, representing the heading followed, is referenced moreover with respect to a graduated scale referenced with respect to north and scrolling along the horizon line. For the pilot, the position of the aircraft symbol on the screen portrays the longitudinal axis of the aircraft at any moment.
For landing, the terrestrial environment reconstructed on the display screen can be supplemented with a representation of the runway whose characteristics are catalogued in landing strip configuration documents which can be accessed by the computer. This artificial representation of a runway is superimposed on the visible real runway when the conditions of visibility are satisfactory. It replaces it when visibility is insufficient.
Moreover, the real direction of movement of the aircraft is different from that of its longitudinal axis, especially on account of sidewind and on account of the fact that the aerodynamic forces which keep the aircraft aloft and its transverse accelerations originate from the tilt of the wing with respect to the direction of movement. This is why the direction of real movement of the aircraft is represented on the screen by a particular symbol generally called the velocity vector. This movement symbol represents the direction of the real velocity vector of the aircraft with respect to the ground; it is defined by two orthogonal components which are on the one hand the drift of the aircraft in a horizontal plane and on the other hand the climb or descent slope of the aircraft with respect to the horizontal plane.
The drift is the angle between the track and the heading of the aircraft, where the direction of the track is defined by the horizontal component of the velocity of the aircraft with respect to the ground, whilst the heading is defined by the direction of the horizontal projection of the axis of the aircraft. Additionally, the climb or descent slope of the aircraft with respect to the ground is defined by an angle whose tangent is the ratio of the vertical component to the horizontal component of the real speed of the aircraft with respect to the ground.
The velocity vector symbol, that is to say the direction of real movement of the aircraft, can be represented on the display screen in a reference frame consisting on the one hand of the moving horizon line, graduated in angular units of heading, and on the other hand of an axis perpendicular to the artificial horizon line, graduated in angles of climb or descent. The velocity vector symbol is placed on the screen at a position referenced with respect to these two axis, as a function of drift (plotted as abscissa along the horizon line) and slope (plotted as ordinate on the axis perpendicular to the horizon line). Drift and slope are computed by the on-board instruments. The pilot can ascertain the drift and the slope at any moment by looking at the position of the symbol with respect to these two axes.
With such a piloting device, the pilot can carry out his manoeuvres by controlling the aircraft movement symbol directly on the display screen in front of his eyes, in particular when visibility is insufficient.
This piloting is further aided by the displaying on the screen, at each instant, of a guidance symbol computed by the computer as a function of a theoretical direction to be followed. Piloting then consists in acting on the controls of the aircraft in a sense which tends to take the movement symbol (or velocity vector) towards the guidance symbol on the screen. When the guidance symbol has the form of a window, piloting consists in trying to keep the movement symbol within the window representing the guidance symbol. Proper guidance therefore depends mainly on the position of the centre of the guidance symbol on the screen, and also on the shape and dimensions of this symbol.
An assistance device which makes it possible to display a guidance window in the case of assistance with landing on a runway equipped with an ILS system (Instrument Landing System) is already known through Patent EP 0 044 777.
In the ILS systems, allowing runway approach in poor visibility, an ideal line of descent is proposed to the vehicle and the deviations between this line and the actual position of the vehicle are measured.
Thus, when the aircraft is moving in such a way that the deviations are constantly zero, the real path of the vehicle coincides with the ideal line.
This ideal line of descent is a line belonging to the vertical plane passing through the axis of the runway and exhibiting an inclination .theta..sub.0 with respect to the horizontal plane of the ground. The inclination .theta..sub.0 is around 2.5 to 3 degrees.
FIG. 1 depicts a view of the vertical plane passing through the axis of the runway. The ideal line of descent 10 belongs to the vertical plane passing through the axis 11 of the runway, it is defined there by its inclination .theta..sub.0 with respect to the horizontal plane of the ground on the one hand and by its intersection with the ground at the ideal point of impact G for landing on this runway, on the other hand. The point G is on the axis of the runway, close to the start of the runway.
In this vertical plane, a measurement of the position P of the aircraft is performed by receiving, on an antenna aboard the aircraft, signals transmitted by a transmitter at G, this measurement E.sub.G, or "Glide deviation", is the difference between the inclination of the ideal line 10 and the inclination of the line 12 joining the projection P.sub.v in this vertical plane of the position P of the aircraft on the one hand and the ideal point of impact G on the other hand.
The horizontal plane parallel to the ground and passing through the position P of the aircraft is represented by the line 17 in FIG. 1, and its contour at infinity represents the 360 degree horizon viewed from the position P.
FIG. 2 depicts a view from above of the runway, the axis 11 of the runway being the line joining the point G situated towards the start of the runway and a point L placed slightly beyond the end of the runway. The projection P.sub.h of the position P of the aircraft in this horizontal plane of the ground on the one hand and the point L on the other hand define a line 20 which deviates by the angle E.sub.L from the axis of the runway. The angle E.sub.L, or "LOC deviation", is measured by receiving, on an antenna aboard the aircraft, signals from a radio transmitter placed at the point L.
The pilot therefore sees the point G at the angle (.theta..sub.0 +E.sub.G) below the horizon line, and the point L at the angle E.sub.L with respect to the heading of the runway.
And if the ILS receiver placed aboard the aircraft indicates a vertical angular deviation E.sub.G or a horizontal angular deviation E.sub.L which is not zero, the aircraft is not on the ideal line of descent.
The known assistance device displays as guidance symbol, a window whose position is defined on the screen from the measurement of the Glide deviation E.sub.G and LOC deviation E.sub.L angles. More precisely, the centre of the window is, on the display screen, at a position which differs from the ideal point of impact G (in the reference frame consisting of the moving horizon line graduated in angular units of heading and of the axis of longitudinal tilts), by amounts which are proportional respectively to the deviations E.sub.L and E.sub.G with the aid of proportionality coefficients k.sub.L and k.sub.G. The pilot must seek to bring the symbol for the real movement of the aircraft into this window and keep it there.
Generally, the datum settings, corresponding to the successive positions of the centre of the guidance window, allow the vehicle progressively to approach the ideal line and hence to align its own path with this line. This constitutes guidance to a line.
The coefficients k.sub.G and k.sub.L regulate the damping of the datum: for small values of these coefficients, guidance towards the ideal line is slow and for higher values, the aircraft is directed more rapidly towards this line.
However, it is observed that when the aircraft is at a position P (P.sub.h, P.sub.v) close to the point G, the taking into account of such a datum takes the vehicle beyond the ideal line and by following the successive datum settings for such guidance, the vehicle begins to oscillate to either side of the ideal line whilst also reducing its distance with respect to the point G. With such guidance, sighting up to the ideal point of impact G is unstable.
In Patent EP 0 044 777, to obtain more stable sighting the gain K.sub.G can vary as a function of the distance to the point G. Moreover, the guidance law is changed in the vertical plane on approaching the ground.